(1) Field of the Invention
The present invention relates to separators used for fuel cells, a method for manufacturing the same, and fuel cells, and more particularly, to a technology for improving the properties of fuel cell separators that operate at relatively low temperatures, including polymer electrolyte fuel cells (PEFCs).
(2) Description of the Related Art
There are several types of fuel cells, including solid oxide electrolyte fuel cells, molten carbonate fuel cells and PEFCs. Solid oxide electrolyte fuel cells and molten carbonate fuel cells operate at a high temperature of 500° C. or above. PEFCs can operate at a relatively low temperature of 200° C. or below by using a solid polymer film made of an ion-exchange resin as an electrolytic film.
Generally, PEFCs have a multi-layered structure in which a number of cells are stacked one against another, with a pair of conductive plates (hereafter referred to as a separator) interposed therebetween. Each cell has an anode on one surface of a solid polymer film and a cathode on the other surface. Each conductive plate has channels for providing fuels such as hydrogen and oxidizers such as air, and ribs formed between neighboring channels. In such PEFCs with a number of cell stacks, power generation efficiency is greatly affected by performance of the separators.
In general, separators are required to have the following properties; high conductivity (low contact resistance), high corrosion resistance, high hydrophilicity, high mechanical strength (high rigidity) and high formability. Also, separators should be thin, light and impermeable to gas. To meet these requirements to a certain extent, carbon materials have conventionally been used for PEFCs.
Carbon materials, however, are not suitable for the development of fuel cells which are smaller but have a high level of output. Reduction in the size of fuel cells requires reduction in the thickness of separators, but since thinner carbon materials lack sufficient mechanical strength and formability, it is impossible to reduce the thickness of the carbon materials limitlessly. In view of this problem, separators having metal substrates have been developed recently, which offer superior mechanical strength and formability with reduced thickness. However, when separators have metal substrates, there arise problems of corrosion resistance and contact resistance. Since metals in general have a low corrosion resistance, separators are easily corroded by water, which is present under reaction in a conventional fuel cell. In addition, a passive layer formed on the surface of a metal by oxidization raises the contact resistance of the separator, to exceed one using a carbon material. This increases a voltage drop for such metal separator substantially when an electric current is fed, which may lead to degradations in the performance of the fuel cell.
To solve this problem, Japanese Laid-Open Patent Application No. H10-228914 discloses the following technique (hereafter referred to as “first conventional technique”). Stainless steel is employed as a metal for the substrate of the separator, and the surface of the separator is plated with a precious metal, such as gold, platinum, or nickel, that has high corrosion resistance and high conductivity. The stainless steel for the substrate has excellent corrosion resistance. In addition, the surface of the substrate is plated with gold or the like that has excellent corrosion resistance and conductivity. This gives the separator a high corrosion resistance and low contact resistance.
However, the plating may suffer the occurrence of pinholes. When the pinholes occur, it becomes difficult to completely cover the substrate. These pinholes can also act as local batteries, accelerating the corrosion of the separator.
A solution to this problem is proposed by Japanese Laid-Open Patent Application No. 2000-164228 (hereafter referred to as a second conventional technique).
FIG. 7 is an enlarged cross-sectional view of the fuel cell separator according to the second conventional technique. As shown herein, the fuel cell separator has a multi-layered structure in which a corrosion resistant layer 3200 and a conductive layer 3300 are formed on the surface of a stainless steel substrate 3100. According to this technique, each layer has its own function, which contributes to raising corrosion resistance and lowering contact resistance of the separator.
However, the second conventional technique has a problem that the corrosion resistant layer 3200 provided on the stainless substrate 3100 shows a high electrical resistance, though it has an excellent corrosion resistance. With such highly electrical resisting corrosion resistant layer 3200 interposed between the substrate 3100 and the conductive layer 3300, even when the corrosion resistant layer 3200 is thin, the electrical resistance of the separator in the direction of the thickness will end up being high.